From: http://www.mssqlcity.com/FAQ/Admin/DBOwner.htm

Answer:

You can use the sp_changedbowner system stored procedure to change the database owner.
Read about the sp_changedbowner stored procedure in SQL Server Books Online.
This is the example to make the user John the owner of the pubs database:

USE pubs
GO
EXEC sp_changedbowner ‘John’
GO